1. Field of the Invention
The present invention relates to an air intake side secondary air supply system for an internal combustion engine equipped with exhaust gas recirculation (EGR) system.
2. Description of Background Information
In an internal combustion engine provided with a three-way catalytic converter in the exhaust system, the air/fuel ratio of the engine is controlled around a stoichiometric value (14.7:1 for example) in accordance with the composition of exhaust gas and the operational conditions of the engine in order to provide an optimum operation of the three-way catalytic converter. The so called air intake side secondary air supply system is an example of the air/fuel ratio control system of this type which has a secondary air passage leading to a portion of an intake air passage downstream of the throttle valve and in which the amount of the secondary air passing therethrough is controlled by varying the sectional area of the secondary air passage so as to adjust the air/fuel ratio of the mixture to be supplied to the engine.
In this secondary air supply system, an air control valve is disposed in the secondary air supply passage, for varying the sectional area of the secondary air supply passage. This is performed by controlling the opening degree of the air control valve in accordance with the pressure level in a pressure chamber of the air control valve. In this case, the pressure chamber is supplied with a control pressure which varies with the operational conditions of the engine. Further, the actual air/fuel ratio is detected from an oxygen concentration of the exhaust gas, and the secondary air is supplied, via the air control valve, to the downstream of the throttle valve, for example by controlling the control pressure, when the detected air/fuel ratio is rich and the supply of the secondary air is stopped or the amount of the secondary air is gradually reduced when the detected air/fuel ratio is lean.
In addition, it is also necessary to decrease the combustion temperature in the engine cylinders in order to prevent the formation of NOx (Nitrogen Oxides) which is one of noxious component contained in the exhaust gas of the engine. Therefore, some internal combustion engines are provided with an exhaust gas recirculation (EGR) system for reducing the combustion temperature, which recircultates a part of exhaust gas into the intake air passage to slow down the combustion process. The exhaust gas recirculation system generally includes an EGR passage connecting the exhaust passage and the intake air passage, and a vacuum operated EGR flow control valve disposed in the EGR passage for varying the sectional area of the EGR passage in accordance with the level of the pressure applied to a pressure chamber thereof. The pressure chamber of the EGR flow control valve is generally applied with a control pressure which varies with the operational conditions of the engine.
In the event that the air intake side secondary air supply system is provided to an internal combustion engine equipped with the EGR system, it is desirable to control the opening degree of the air control valve and the EGR flow control valve by supplying a control pressure from a single source of the control pressure, which varies with the operational conditions of the engine, an amount of the intake air for example. This is very important for the simplification of the total construction of the peripheral systems of the internal combustion egine and the reduction of the production costs.
However, if the pressure chambers of the air control valve and the EGR flow control valve are provided with the same control pressure from the single source of control pressure, then there is a fear that the combustion temperature is excessively reduced in some conditions, to adversely affect the performance of the engine. More concretely, the amount of EGR may be increased during a period in which the air/fuel ratio is controlled to the lean side by supplying the air intake side secondary air after a detection of the rich air/fuel ratio.